Which original U.S. state was last to ratify the Constitution, on May 29, 1790?

Rhode Island was the last of the original U.S. states to ratify the Constitution, doing so on May 29, 1790.

Rhode Island initially rejected the Constitution in a statewide referendum in March 1788. Critics feared that the proposed federal government would threaten state sovereignty and individual liberties. The state eventually held a ratifying convention in Newport, where delegates approved the Constitution by a vote of 34–32.

By the time Rhode Island ratified, the Constitution had already taken effect. Article Seven required ratification by nine states, a threshold reached when New Hampshire approved the document on June 21, 1788. North Carolina had ratified in November 1789, leaving Rhode Island as the sole holdout among the original thirteen states.

Rhode Island's delay did not prevent it from becoming part of the Union under the Constitution. It was the thirteenth and final original state to ratify. Vermont, which had not been one of the original thirteen, joined the Union separately in 1791.